An automobile body shell has a monocoque structure which is obtained by joining many formed panels and structural members, which are produced by press-forming a sheet metal to each other. A center pillar (B pillar) is known as one of these structural members. The center pillar is an important structural member that is disposed between a side sill (kicker) forming the body side and a roof rail. The center pillar (a) supports the locking mechanism of a front door and supports a rear door in a freely openable/closable manner, (b) ensures the flexural rigidity and torsional rigidity of the body shell during the automobile running, and (c) ensures a survival space for passengers in a lateral collision. Usually, the center pillar is formed by joining a center pillar outer panel, a center pillar inner panel, and a center pillar reinforcement, which is a reinforcing member disposed between these panels (for example, by spot welding the three lapped sheets).
The center pillar reinforcement includes a body that is lengthy and curved and two substantial T shape sections formed at both end portions in the longitudinal direction (up-to-down direction) of the body, and therefore the center pillar reinforcement is a formed panel taking a substantially I-form outer shape in a plan view.
The body has a cross-sectional shape such as a substantially hat shape formed by a curved and lengthy top sheet, two vertical walls formed so as to connect with both edge portions in the width direction of the top sheet, and outward flanges formed so as to connect with the respective two vertical walls.
Of the two substantial T shape sections, the substantial T shape section disposed in the upper portion of the center pillar reinforcement is joined to any other framework member such as a roof side rail reinforcement, and the substantial T shape section disposed in the lower portion of the center pillar reinforcement is joined to any other framework member such as a side sill outer reinforcement. To assure the joint strength and rigidity of these substantial T shape sections, each of these substantial T shape sections has a top sheet, two curved vertical walls connecting with the top sheet, and outward flanges formed so as to connect with each of the two vertical walls.
The center pillar reinforcement is required to have the above-described complicated shape, to have a high strength, for example, at least 400 MPa as a reinforcing member, and further to be capable of being produced at a low cost.
If it is attempted to produce the center pillar reinforcement by press-forming of a blank sheet of metal, there arises a problem that wrinkles and cracks are liable to occur on the top sheet and flanges in the upper end portion and the lower end portion of the substantial T shape sections. The strength of blank is the higher, this problem is the more remarkable.
To prevent the occurrence of this problem, the center pillar reinforcement is typically produced by drawing press-forming. To perform the drawing press-forming, the metal blank for the center pillar reinforcement is required to have a very high extensibility. Therefore, as the metal blank for the center pillar reinforcement, a material having a relatively low-strength (for example, a steel sheet having a tensile strength of about 340 MPa) can only be used. In order to ensure the required high strength, it is necessary to use a thick and heavy blank. Therefore, the weight of automobile body shell increases, and the production cost rises.
Also, in order to produce the center pillar reinforcement by means of drawing press-forming, the blank needs to have a wide margin thickness (cut-off portion) because the outer peripheral portions of blank are strongly restrained by a blank holder. Thereby, the production cost of center pillar reinforcement further increases.
In addition, some center pillar reinforcements cannot be monolithically molded due to the shape and strength thereof. These center pillar reinforcements need to be formed by joining a plurality of components that are press-formed separately. For this reason as well, the production cost of center pillar reinforcement increases.
Patent Documents 1 to 4 disclose inventions of manufacturing press-formed products each having a simple cross-sectional shape such as a simple hat shape or a Z shape are produced by various types of bend-forming. However, Patent Documents 1 to 4 do not disclose a method for producing the center pillar reinforcement having a high strength and a complicated shape as described above at a low cost.